TETRAGONAL-ORTHORHOMBIC PHASE-TRANSITION IN YBACUO THIN-FILMS OBSERVED BY PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY

Oxygen-deficient, tetragonal thin films of YBa2Cu3O6+x with x approxim ate to 0.25, quenched from the deposition temperature, change to the o xygenated, orthorhombic phase with x approximate to 1, between 200 deg rees C and 400 degrees C in flowing oxygen. The transition is not reve rsible in flowing oxygen, and cannot be completely reversed by cooling in flowing argon. We do not observe a transition of the orthorhombic films to the tetragonal phase up to 800 degrees C in flowing oxygen. W e observe that the major impurity phases to appear under nonoptimal an nealing conditions are oriented phases of YCuO2 and BaCu2O2 with Y2BaC uO5 and Y2Cu2O5 conspicuously absent. These conclusions have been draw n from a study that uses perturbed angular correlation spectroscopy to probe the local microstructure of the films.